In general, after being mixed into an aqueous or solventborne resin system, an organic pigment must be further dispersed prior to its final application in order to ensure that the organic pigment is homogeneously dispersed in the resin system. This additional dispersion step generally requires that the pigment be dispersed for a period of 2 to 48 hours using milling equipment, such as a vertical or horizontal bead mill or an attritor mill with milling media, such as glass beads or stainless steel balls. Since this additional dispersion step is both time-consuming and costly, the elimination of this step by using pigments that are adequately dispersed during a simple mixing step, without the need for the additional dispersing step, is a great advantage.
Pigments which are substantially homogeneously dispersed in the high-molecular-weight organic material without the above-described additional dispersion step are referred to as "stir-in pigments" in this application.
It is generally known that "effect" pigments, which often are relatively large particle size, platelet-shaped particles, are usually incorporated in coatings systems without an additional dispersion step. The additional dispersion step is avoided in order to prevent the breaking of the large platelet-shaped particles.
In this application, the expression "effect pigment" means inorganic or organic pigments which show metallic, pearlescent and/or silky-luster effects. Such effect pigments are generally inorganic pigments such as metallics, like aluminum, TiO.sub.2 -coated mica pigments, platelet graphite and platelet molybdenum disulfide. Other effect pigments are prepared by coating a flaky crystalline form of a substrate with a small amount of a dyestuff or inorganic pigment, for example, a metal oxide coated mica. In addition, the effect pigments include certain platelet-shaped organic pigments, such as platelet copper phthalocyanine and those described in U.S. Pat. No. 5,084,573, U.S. Pat. No. 5,095,122, and allowed U.S. Pat. No. 5,347,014. In each instance, such pigments are readily distinguished from conditioned organic pigments; especially by their particle size and specific surface area.
U.S. Pat. No. 5,298,076 discloses the use of an unconditioned carbazole dioxazine crude as a multi-colored effect pigment for use in a variety of applications, including as a stir-in pigment. This publication does not suggest that a conditioned carbazole dioxazine pigment could be utilized as stir-in pigment.
A number of conditioning processes are known for the conditioning of organic pigments by diminution of the corresponding crude organic pigment. For example, milling and kneading in the presence of salt, or wet milling processes in the presence of appropriate additives, are well-known conditioning processes for organic pigments. However, it has not been disclosed that such conditioned organic pigments can be used as stir-in pigments.
This invention relates to the surprising discovery that a pigment which has excellent stir-in pigment properties is prepared by wet-milling a pigment crude consisting of primary pigment particles with an average particle size of from 3 to 35 .mu.m to yield an organic stir-in pigment with an average particle size of 0.1 to 9 .mu.m. Since the organic stir-in pigments of this invention are simply added and stirred into a suspension or solution of the high-molecular-weight organic material, the inventive stir-in pigments avoid the costly energy and time consuming dispersion process that is normally required to uniformly disperse pigments in a coating or ink system.